They worry that Mr. Trump will make the same kinds of concessions to Mr. Putin when they meet in Helsinki, Finland, on July 16 that he made to Mr. Kim in Singapore, tilting a relationship that has already swung in Russia’s favor.

In the past few weeks alone, Mr. Trump has called for Russia to be readmitted to the Group of 7 industrial powers, suggested it has a legitimate claim to Crimea because a lot of Russian speakers live there and continued sowing doubts about whether Moscow meddled in the 2016 presidential election — or if it did, whether the sabotage actually benefited Hillary Clinton.

In Singapore, Mr. Trump emerged from a lunch of sweet and sour crispy pork with Mr. Kim to declare he had solved the nuclear crisis with North Korea, even though the North conceded nothing on its weapons and missile programs. Mr. Trump also canceled joint military exercises with South Korea, a concession long sought by Pyongyang.

It has become a recurring motif for Mr. Trump as a statesman: In November, he lavished praise on President Xi Jinping of China after a one-on-one meeting in Beijing, during which Mr. Xi offered no concrete concession on trade — an issue that matters more to Mr. Trump than almost any other.

What these three leaders have in common is that they are autocrats, whom Mr. Trump admires and believes he can win over with a brand of personal diplomacy that dispenses with briefing papers or talking points and relies instead on a combination of flattery, cajolery and improvisation.

“Trump sees a good meeting as a positive diplomatic achievement,” said Michael McFaul, a former American ambassador to Moscow. “That’s wrong. Good meetings are a means to an end.”

Given Russia’s record of malfeasance — from its annexation of Crimea to its interference in the American election — Mr. McFaul said, “Trump should not praise Putin and signal a desire to just move on.”

“That does not serve American national interests,” he said.

Secretary of State Mike Pompeo and other officials insist that the Trump administration has been tougher toward Russia than the Obama administration was. They cite American sanctions, arming of Ukrainian troops, expulsion of diplomats and vocal public shaming of Russia for its cyberattacks.

But even if that is true — and former Obama officials dispute it — Mr. Trump’s steadfast refusal to criticize Mr. Putin has largely vitiated these measures.

“What matters is what the president says,” said R. Nicholas Burns, a diplomat who served in the Clinton and George W. Bush administrations. “And what he’s been saying completely undermines the policy. It’s like he’s untethered from his own administration.”

Even those who credit the president for taking steps that Mr. Obama did not, like sending lethal weapons to Ukraine, say those measures are largely lost in the mystifying embrace of Mr. Putin by Mr. Trump.

“The administration’s staffers are focused on the lyrics,” said Richard N. Haass, the president of the Council on Foreign Relations. “What they are ignoring is the music. Some of the lyrics are tough, but the music is a love song.”

Mr. Trump argues that his personal relationships will ultimately yield results that have eluded his more conventional predecessors. In an interview with Fox News after the Singapore summit meeting, he said that if he could have dinner with Mr. Putin, he could persuade him to withdraw from Syria and stop preying on Ukraine.

“I could say: ‘Would you do me a favor? Would you get out of Syria?’” Mr. Trump said. “‘Would you do me a favor? Would you get out of the Ukraine?’”

Mr. Pompeo, testifying Wednesday before Congress, said Mr. Trump would raise the issue of Russia’s election meddling with Mr. Putin. But the last time he did that, at a regional summit meeting last fall in Asia, he told reporters that he believed that Mr. Putin’s denials of Russian involvement were sincere.

“Every time he sees me, he says, ‘I didn’t do that,’ and I really believe that when he tells me that, he means it,” Mr. Trump said of Mr. Putin. “I think he is very insulted by it, which is not a good thing for our country.”

Mr. McFaul predicted Mr. Putin would be a formidable counterpart in a one-on-one meeting, well briefed on American foreign policy and determined to use that knowledge to undermine the administration’s policies, especially on Ukraine.

A common thread in Mr. Trump’s approach to autocrats, Mr. Haass said, is that he views these relationships as having no history, no baggage that constrains how these leaders may act. They are personal encounters, with none of the trade-offs or compromises that usually characterize summit meetings.

Mr. Trump is not the first president to put a premium on building rapport with Russian leaders. Bill Clinton did it with Boris Yeltsin; Mr. Bush tried to do it with Mr. Putin; Barack Obama cultivated Dmitri A. Medvedev, who served as president between Mr. Putin’s terms. But only Mr. Trump has made it virtually his entire agenda.

That is not to say that he and Mr. Putin lack meaty issues to discuss. Some analysts speculated that Mr. Trump would look for common ground on Syria. During the transition, his aides had considered lifting sanctions on Russia in return for Russia’s cooperation with the United States against Iran in Syria. The idea fell to the wayside, but some wonder whether the president could revive it.

“Is Trump trying to get Putin to move against the Iranians in Syria?” said Martin S. Indyk, a former American ambassador to Israel. “And if so, what price is he prepared to pay for that?”

Such a proposal would make Mr. Trump’s meeting with Mr. Putin more substantive than those with Mr. Kim or Mr. Xi. But persuading Russia to change its strategy in Syria would be hard, and lifting sanctions on Moscow would cause another rift with European allies already bruised by Mr. Trump. It would, in short, require the president to engage in the diplomatic heavy lifting he has so far shunned.

As Henry A. Kissinger, the former secretary of state, once wrote, “It is dangerous to rely on personality or negotiating skills to break deadlocks; they cannot redeem the shortcomings of an ill-considered strategy.”

A version of this article appears in print on , on Page A1 of the New York edition with the headline: In Russian Talks, Experts Fear U.S. Will Give Up Too Much. Order Reprints | Today’s Paper | Subscribe